1. Field of the Invention
The present invention relates, in general, to a method for measuring biological oxygen demand (hereinafter referred to as "BOD") continuously and rapidly, and an apparatus therefor. More particularly, the present invention relates to a method with which BOD can be automatically measured in 20 min by using the apparatus.
2. Description of the Prior Art
BOD refers to the oxygen amount required to oxidize the organic materials which can be decomposed by the metabolic action of aerobic microorganisms. Because BOD is indicative of the amount of the organic material present in a solution, BOD is one of the most important indicators for water pollution and has the following implications.
When sewage or waste water is discharged from homes or plants to a river, the organic materials contained therein are primarily decomposed by the aerobic microorganisms which inhabit the river. Owing to this, the dissolved oxygen in the river is exhausted, thereby creating an environment in which it is very difficult for higher aerobic organisms, such as fish, to survive.
Thus, the BOD of sewage or waste water is indicative of the concentration of the organic materials it contains, and the influence of the sewage or waste water on the environment. Most countries of the world have their own BOD allowance limits in order to protect their own environments.
To reduce the deleterious influence of waste water, the organic materials in the waste water should be removed prior to being discharged into the environment. One method is to biologically treat the waste water. In fact, most home sewage and plant waste water is processed by such a biological treatment method.
According to this treatment method, the organic materials in waste water are decomposed by a high concentration of aerobic microorganisms (active sludge), prior to being discharged into the environment. This is based on the strategy that a phenomenon occurring in nature is intensively and rapidly utilized at a safe site before the sewage or waste water is discharged into the environment.
In running a process for the biological treatment of waste water, the BOD of an influx and the BOD in an aeration bath are very important parameters to normally run the process and to prevent various problems which may occur in the process. Further, because the BOD of an efflux, as mentioned above, is legally regulated, it is continuously measured and managed.
Therefore, in order to meet legal regulations or in order to smoothly run the biological treatment process, the BOD of waste water should be measured.
Conventionally, the measurement result is completely interpreted 5 days after measurements are taken. This delay hinders the application of the BOD result to waste water treatment process management. The delay also makes the BOD result unreliable in legally regulating the waste water of a plant.
There is a standard for BOD measurement in each country. For example, such standards include American Public Health Association Standard Method No. 219 in the U.S.A., Japanese Industrial Standard JIS K0102-1974 in Japan, and KS M0111 #19 in Korea. These three are almost the same in practice and all of them give complete interpretation of BOD 5 days after BOD measurement. In addition, because these measurement processes are very difficult and complicated, the BOD results are modulated somewhat, depending on the skill of the experiment workers. Further, even though skilled workers measure BOD, the results are not good in reproducibility.
With increasing interest in the environment, a new BOD measurement method that is fast, correct and displays good reproducibility is desperately required.
Up to now, much effort has been made to solve the problems of conventional BOD measurement techniques. For example, U.S. Pat. No. 4,350,763 to Shuichi Suzuki (hereinafter referred to as "'763 patent") suggests a quick method by which BOD can be measured in 30 min. The quick BOD measurement of the '763 patent has a significant advantage of being quicker and simpler than a conventional BOD measurement (hereinafter referred to as "BOD5"). According to the '763 patent, microorganisms are immobilized on a dissolved oxygen (hereinafter referred to as "DO") sensor membrane. In solutions free of organic material, constant DO values are read by their action. When a solution at a constant flow rate and containing organic material comes into contact with the immobilized microorganisms, the microorganisms use the dissolved oxygen to decompose the organic material. The DO value decreases with the concentration of the contained organic material. In other words, '763 patent takes advantage of the fact that the concentration of organic material in a sample is proportional to .DELTA.DO (difference in DO between an organic material-free buffer solution and a sample).
The immobilized-microorganism sensor of '763 patent is advantageous in that it is simple, but it has the following disadvantages:
First, because the activity of the microorganisms immobilized on the DO sensor membrane changes with time, frequent reference to a standard solution (defined in BOD) should be made in order to detect the .DELTA.DO, thereby calculating the BOD.
Second, measurement itself is impossible unless the organic material in the sample is decomposed by the microorganisms. This phenomenon occasionally occurs in the waste water from plants.
Third, when the concentration of the organic material in a sample exceeds a certain degree, the oxygen consumption rate of the microorganisms does not increase any more. In such case, the concentrated sample should be diluted to a concentration at which the organic material concentration and the oxygen consumption rate are in a proportional relation, in order to be able to measure the BOD of the sample.
Another prior technique is disclosed in U.S. Pat. No. 4,898,829 to Friedrich W. Siepmann (hereinafter referred to as the "'829 patent") which complements the above-noted disadvantages of the '763 patent. According to the '829 patent, carriers on which microorganisms are immobilized are placed in a reactor into which test water continuously flows, so that the microorganisms capable of decomposing the organic materials in the test water naturally adhere to the carriers and grow therein. This addresses a disadvantage of the '763 patent--the indecomposibility of organic material.
The '829 patent is similar to the '763 patent in measurement principle but significantly different in working mechanism. In accordance with the '829 patent, a sample is aerated to have a saturated DO concentration at a certain temperature and then fed into a reactor. As in the '763 patent, the oxygen consumption rate of the microorganisms changes with the concentration of the organic materials in the sample, so that the sample which is passing through the reactor comes to have a different DO.
To avoid the dilution problem of the '763 patent, the '829 patent suggests an apparatus equipped with a diluting water line with which the sample passing through the reactor has a constantly maintained .DELTA.DO (difference in DO between at the inlet and at the outlet of the bath). When the BOD of a sample is too high, the DO is considerably decreased, which can be compensated for by adding a great amount of diluting water to the sample. On the other hand, a smaller BOD is attributed to a low decrease of DO, which requires the addition of a small amount of diluting water.
The apparatus of the '829 patent further comprises a microprocessor with which the flow rates of the diluting water and the sample are controlled, thereby calculating the BOD of the sample.
The '829 patent is a considerably advanced technique capable of solving the problems of '763 patent including: 1) indecomposibility of organic material, 2) dilution of sample, and 3) maintenance in the activity of immobilized microorganisms, as well as automatic measurement of BOD. The '829 patent is still limited by the following:
First, when the sample has a low concentration of organic material, there are not enough nutrients to grow the microorganisms adherent to the carriers, so that immobilized microorganisms are not maintained to the degree of being able to measure BOD.
Second, because the activity of the microorganisms has a great influence, as in the '763 patent, the activity should be constantly maintained. Following the installation of the apparatus, a waiting period should be given until the microorganisms of the carriers reach a steady state. As such, it takes a long time to become ready to measure the BOD of a sample.
Third, the measurement method should be of a continuous type requiring continuous inflow of test water. Because the flow rates of the sample and the diluting water are controlled with maintenance of constant delta DO, it is impossible to apply a laboratory scale measuring apparatus other than a continuous measuring apparatus working at the site.